How to Get Measurements to Cut Roof Rafters

Use a framing square, tape measure and pencil to measure and mark rafters. Get a house plan, if possible, to provide basic information: the width or span of the roof; the rafter run, which is half the span; and the pitch or angle of slope from wall to peak expressed in inches of rise per foot from wall to peak, such as 5/12 for a slope of 5 inches per foot.

Start with a 16-foot, 2-by-4-rafter board for most moderate pitch roofs; the actual length needed will vary with the size of the roof. Lay the rafter board on a flat surface. Put the point or heel of the framing square at the bottom of one end. Align the pitch mark on the thin tongue and the 12-inch mark on the wide blade at the top of the board; for a 5/12 that would be the 5-inch mark. Mark the angle that forms along the tongue; that is the top or plumb cut.

Look up the "length of common rafters per foot of run" table on the square's blade. Find that length under the pitch mark; for a 5-pitch it will say 13, or 13 inches of rafter for every foot of run. Multiply 13 by the run, 12 feet for a 24-foot wide roof, for instance; that comes to 156 inches. Measure with a tape measure 156 inches down the bottom of the board from the bottom of the plumb cut and mark that spot.

Draw a 1-inch line vertically up into the bottom of the rafter. Measure back up the board from the 156-inch mark 3 1/2 inches and mark that spot. Connect that mark with the top of the vertical line to create a triangle, called a birdsmouth, which will fit over the cap board on the wall.

Measure down from the end of the birdsmouth a specified overhang or eave, typically 18 inches on a house roof; that will be 174 inches or 14 1/2 feet. Mark that spot and draw another angle, like the top cut but reversed, by putting the heel of the square at the top of the board instead of the bottom. That completes measuring and marking a rafter.